S1

Supporting information

Exploration of VPO4 as a new anode material for sodium-ion

batteries

Xinghui Liang,a Xing Ou,a,* Hong Dai,b Fenghua Zheng,a Qichang Pan,a Peipei Liu,a

Xunhui Xiong,a Meilin Liu c and Chenghao Yang,a,*

a. Guangzhou Key Laboratory for Surface Chemistry of Energy Materials, New

Energy Research Institute, School of Environment and Energy, South China

University of Technology, Guangzhou 510006, P. R. China.

b School of Materials Science and Engineering, Central South University, Changsha,

410083, P.R. China

c School of Materials Science & Engineering, Georgia Institute of Technology,

Atlanta, GA30332-0245, USA

E-mail: esouxing@mail.scut.edu.cn, esyangc@scut.edu.cn

Electronic Supplementary Material (ESI) for ChemComm.This journal is © The Royal Society of Chemistry 2017

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Experimental Section

Synthesis of VPO4@C composite.

Stoichiometric ratio (1:1:0.2:0.5) of vanadium (III) acetylacetonate (C10H14O5V),

ammonium phosphate ((NH4)2HPO4), glucose (C6H12O6) and oxalic acid (C2H2O4)

were sequentially dissolved in deionized water under continuous stirring and heating.

The C10H14O5V was added as vanadium source and carbon source simultaneously.

After forming a sol precursor, then the mixed solution was continuous stirred and

heated at 80°C to achieve the gel, followed by drying in a vacuum oven at 100 ˚C

overnight (freeze drying). Finally, the as-prepared powders were obtained by sintering

at 300oC for 3 hours and 900°C for 1 hours in Ar/H2 (95:5) atmosphere.

Material characterizations

X-ray diffraction (XRD) was collected with Bruker Advance D8 diffractometer

using Cu Kα source. Thermogravimetric analysis (TGA) was carried out by a Mettler

Toledo TGA/DSC-1100. Raman test was conducted by a LabRAM Aramis

spectrophotometer with a laser wavelength of 532 nm. X-ray photoelectron (XPS)

was recorded on a LabRAM Analyzer with wavelength of 514 nm. Scanning electron

microscopy (SEM) and Transmission electron microscopy (TEM) were performed on

JEOL JSM-7500FA and JEOL-2011 at 200 kV equipped with energy-dispersive X-

ray spectroscopy (EDS), respectively.

The in-situ XRD measurement was recorded by home-design cell and as reported

previously.S1,S2 The cell was made of stainless steel and inset with an internal slot

with 12-mm diameter, while the configuration was illustrated in Fig.S1. Specifically,

S3

the beryllium foil served as transition window to allow X-ray passage, and the carbon

paper acted as a current collector. The electrode was obtain by mixing the active

material (VPO4/C) and polyvinylidene fluorid binder (PVDF) with ratio of 8:2, which

were homogeneously dispersed by the N-methylpyrrolidinone (NMP) solvent and

then cast into the carbon paper. Each XRD pattern was performed in the step

incremental of 0.02° and scaned between 2=10°-60° at the rate of 0.08° s-1. There

was 30 s for interval for each required pattern. The corresponding charge/discharge

measurement was carried out at the rate of 100 mA g-1, ensuring that at least 40 scans

can be recorded for a entire sodiation/desodiation cycle.

Electrochemical measurements

The as-prepared electrodes were prepared by mixing VPO4 (70%), carbon black

(20%), and PVDF (10%) in NMP solvent. The slurry was coated on copper foil,

followed by drying at vacuum oven. The electrodes were punched into round disks

with 1.0-1.2 mg cm-2 loading of active material, and then assembled in the glovebox

by sodium tablet, glass fiber (Whatman GF/D) and 1.0 M NaCF3SO3 dissolved in

diethylene glycol dimethylether (DEGDME) as reference/counter electrode, separator

and electrolyte, respectively. The galvanostatic tests were conducted on a Land

System (CT2001A) in the potential range of 0.01-3.0 V. Cyclic voltammogram (CV)

at a scan rate of 0.1 mV s-1 and electrochemical impedance spectroscopy (EIS)

measurements over the frequency range from 100 kHz to 0.01 Hz were performed on

a CHI660E electrochemical work-station and IM6 (Zahner) electrochemical station,

respectively.

S4

Fig. S1. The configuration of in-situ battery cell.

Fig. S2. Rietveld refinement XRD pattern of bare VPO4 (A) and VPO4@C (B).

S5

Table S1. Experimental lattice parameters calculated from the Rietveld refinement for

bare VPO4 and VPO4@C composites.

Samples a (Å) b (Å) c (Å) V(Å3) Rwp Rp

VPO4 5.2212093 7.7634015 6.2760865 254.39705 25.67 20.13

VPO4@C 5.2284616 7.7828670 6.2657795 254.96974 20.32 16.59

Table S2. Structural parameters calculated from the Rietveld refinement for bare

VPO4

Atom site x y z Occ.

V1 4a 0.00000 0. 00000 0.00000 1

P1 4c 0.00000 0. 35055 0.25000 1

O1 8g 0.24720 0.47002 0.25000 1

O2 8f 0.00000 0.24518 0.03811 1

Table S3. Structural parameters calculated from the Rietveld refinement for the

VPO4@C

Atom site x y z Occ.

V1 4a 0.00000 0. 00000 0.00000 1

P1 4c 0.00000 0. 35497 0.25000 1

O1 8g 0.24826 0.46801 0.25000 1

O2 8f 0.00000 0.24148 0.04237 1

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Fig. S3. TGA measurement of bare VPO4, and VPO4@C composites in the

temperature range of 30-700 °C in the flowing of air atmosphere (A), XRD patterns

for final product of VPO4 sintered at 700 °C under air atmosphere (B).

TGA test is operated in air flow to calculate the carbon content of VPO4@C (Fig.

S3A). The apparent increasing mass of bare VPO4 is corresponded to the oxidation of

VPO4 to VOPO4 (Fig. S3B). In contrast, the rapid mass loss starting from 400°C for

VPO4@C sample is related to the removal of carbonous materials.

Fig. S4. Raman spectrum of bare VPO4 and VPO4@C composites.

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Fig. S5. XPS full pattern (A) and core level of V 2p (B) for VPO4@C composites.

Fig. S6. XPS core level of V 2p (A) and P 2p (B) for bare VPO4.

It is clearly observed that the binding energy of P 2p spectrum for bare VPO4 is

133.9 eV, which is higher than that of VPO4@C composite. The difference of P 2p

spectrum between VPO4 and VPO4@C is ascribed to that the P 2p spectrum of bare

VPO4 can only be fitted into one peak, which is assigned to the P-O bond. The result

is consistent with other literatures.S3,S4

S8

Fig. S7. N2 adsorption–desorption isotherms and (inset) the pore size distributions of

bare VPO4, and VPO4@C composite.

According to the Brunauer-Emmett-Teller (BET) measurements, the

corresponding specific surface area of the VPO4@C is 13.51 m2 g-1, which is larger

than that of VPO4 (6.22 m2 g-1). For VPO4@C composite, the larger surface area with

porous structure is beneficial to Na+ transportation and electrolyte diffusion.

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Fig. S8. SEM (A, B), TEM and (C) HRTEM (D) images of VPO4.

Fig. S9. The CV plots of VPO4 in the first 4 cycles between 0.01 V and 3.0 V at a

scanning rate of 0.1 mV s-1.

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Fig. S10. The charge/discharge curves of VPO4 in the first 4 cycles between 0.01 V

and 3.0 V at rate of 50 mA g-1.

Fig. S11. Performance comparison of VPO4@C with other recently reported anodes

for SIBs

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Table S4. Comparison of electrochmical performance for VPO4@C with other

materials reported elsewhere as the anode materials for SIBs.

Sample

Rate capability

Capacity/current

(mAh g-1/mA g-1)

Cycling stability

Capacity/current/cycles

(mAh g-1/mA g-1/n)

Reference

s

VPO4@C 204.8/2000 245.3/1000/200 This work

C 100/2000 160/100/100 S5

TiO2 82.7/2000 160/20/50 S6

V2O5 140/1280 177/40/100 S7

NaV3(PO4)3 107/2200 126/220/100 S8

Na3V2(PO4)3 103/117 136/12/50 S9

NaTi2(PO4)3 85/2660 77/1330/1000 S10

Na2Ti3O7 71/885 125/35.4/50 S11

NaAlTi3O3 65/250 62/25/100 S12

Na2.65Ti3.35Fe0.65O9 74.2/100 110/40/100 S13

Na2/3Ni1/6Mg1/6Ti2/3O2 41/96 80.4/9.6/100 S14

S12

Fig. S12. The recorded impedance spectra of VPO4 (A) and VPO4@C (B) before and

after various cycles, linear fitting to Z’ versus ω-1/2 plots in the low-frequency range

(C), equivalent circuit used for fitting the experimental EIS data (D).

Table S5. Result of electrochemical impedance and Warburg coefficient in Figure

S13.

Samples Cycle number, n Rs, Ω Rct, Ω σw, Ω s-1 DNa, cm2 s-1

Pristine 13.01 4.26 235.5 8.19×10-15

10 14.83 19.1520 14.98 64.52VPO4

50 14.68 93.25Pristine 8.61 2.58 138.9 2.35×10-14

10 8.97 4.1520 8.84 5.46VPO4@C

50 9.32 14.17

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Fig. S13. Selected 2 regions plot of in-situ XRD results of VPO4/C electrode against

the voltage profile during the initial cycle.

For in-situ XRD measurement, the cell was galvanostatic charged/discharged at a

current rate of 100 mA g-1, while each XRD patterns were collected at different

desodiation/sodiation state in the first cycle and stacked together sequently.

Meanwhile, the coresponding contour plot of in-situ patterns is displayed in Fig. 4A.

The phase compositions are color-coded to have a better distinction for

desodiation/sodiation process. The red color means low intensity, and the blue color

means high intensity, which are shown in the right side of Fig. 4A. It is better to

observe the phase transformation by combination of charge/discharge curves and

XRD pattern at various desodiation/sodiation states.

The intermediate phase is a new material with peaks located at 2θ of 24o and

30.5o, which are neither Na3PO4 nor VPO4, probably ascribed to the combination of

Na+ and VPO4 and formation of NaxV(PO4)y. The NaxV(PO4)y could be indexed into

the Na3V3(PO4)3 or Na4V2(PO4)3, which can be found in the other polyanion anode

materals of sodiated NaV3(PO4)3 S15,S16 and Na3V2(PO4)3,S17,S18 respectively. However,

the intermediate NaxV(PO4)y is metastable and hardly to determine the accurate

S14

structure, and this phenomenon usually occurs in the halfway of sodiation/desodiation

process.S15-S18 Under the limit of experiment condition, other characterizations should

be carried out to verify the intermediate phase.

S15

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